<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        var a = 10;
        ++a;//++a  11   a=11
        var b = ++a + 2;//a=12   ++a = 12
        console.log(b);//14
        var c = 10;
        c++;//c++  11   c=11
        var d = c++ + 2;//c++ = 11  c=12
        console.log(d);//13


        var e = 10;
        var f = e++ + ++e;//e++ = 10 e=11   ++e  12   e=12
        console.log(f);//22


        // 后置自增  先表达式返回原值  后面变量在自加1 

    </script>
</head>
<body>
    
</body>
</html>